Previous investigations from our laboratory have shown that heart rate-dependent and intermittent block processes, such as during Wenckebach periodicity in AV transmission, cannot be explained merely on the basis of slow conduction, and are not compatible with the behavior expected from an homogeneous system of electrically coupled cells. It is our hypothesis that, in a nonhomogeneous system that involves an area of conduction impairment separating two fully excitable zones, the success or failure of propagation at a given rate is determined by the recovery of excitability of the cells beyond the impairment zone. Our goal is to investigate in single cells the cellular and subcellular mechanisms of time-dependent changes in excitability that are responsible for cyclic conduction block patterns. We will study the role of current source properties and intercellular communication between cell pairs in determining success or failure of impulse propagation, as a function of driving frequency. We will use current and voltage clamp techniques in enzymatically dissociated guinea pig ventricular myocytes, as well as computer simulations toward the following Specific Aims: 1) Study the passive membrane properties of single cells and characterize the changes in their excitability during diastole. 2) Study the ionic mechanisms responsible for post-repolarization refractoriness and rate-dependent failure of activation. 3) Investigate the roles of action potential amplitude and maximum upstroke velocity as source properties in determining action potential propagation. 4) Correlate junctional resistance with action potential propagation between pairs of coupled myocytes. 5) Study the development of newly-formed intercellular communications between single heart cells. These studies should improve our understanding of the factors involved in the property of excitability of cardiac myocytes and their ability to conduct electrical impulses. The results should give also precise and direct answers about the ionic bases of conduction block processes with alternation or with Wenckebach periodicity.